Decorative hard coatings with a variety of color appearances have been introduced to the market in at least the past twenty years for surface finishing in innumerable applications, for example, in house-hold appliances, kitchen/bathroom accessories, door accessories, automobile parts, watches, jewelery, and many other articles and apparatuses. The most successful decorative hard coatings are TiN, TiCN, ZrN and ZrCN because such coatings can simulate a variety of gold colors and at the same time possess a Vickers hardness of about 20 GPa. The production of such coatings by physical vapor deposition is well known. (See, for example, Johnson, P. C., “The Cathodic Arc Plasma Deposition of Thin Films”, in Thin Film Processes II edited by Vossen & Kern, Academic Press Inc., San Diego, USA, 1991, pp. 209-280.)
Coatings have also been developed that have colors other than gold. For example, decorative black hard coatings exist that are discussed in U.S. Pat. No. 4,524,106 and U.S. Pat. No. 4,758,280. Hard coatings with a bright metallic white appearance also exist. Bright metallic white hard coatings are dominated in the marketplace by chromiun coatings and nickel coatings, which are produced most practically by electrochemical plating techniques. (See, for example, ASM Metals Handbook, Volume 5 Surface cleaning finishing and coating, 9th ed., ASM, Ohio, 1982; and ASM Specialty Handbook—Tool Materials, Davis, J. R., ed., ASM, Ohio, 1995.)
The dominant chromium and nickel coatings can only reach a Vickers hardness of about 10-11 GPa. Typically, chromium has a light bluish tint and nickel a light greyish tint, and thus these coatings are not as bright metallic white as some more expensive metals such as silver, platinum, rhodium, and palladium. Accordingly, the coatings are sometimes over-coated by the more expensive metals. Furthermore, concerns about environmental pollution have led to a steady reduction in the electrochemical production of hard chromium coatings. As for hard nickel coatings, human skin allergy has practically wiped out their use in the watch and jewellery industries.